Discontinuous high-modulus fiber metal matrix composite for physical vapor deposition target backing plates and other thermal management applications

ABSTRACT

The invention includes the use of a high-modulus fiber metal matrix composite material as a backing plate for physical vapor deposition targets, as a lid for microelectronics packages, as a heat spreader, and as a heat sink. In one implementation, copper-coated carbon fibers are mixed with copper powder. In another implementation, the mixture is consolidated to a carbon fiber metal matrix composite by using a vacuum hot press. The resultant backing plate has a coefficient of thermal expansion of 4.9×10 −6 /C, thermal conductivity of at least 300 W/mK, density of greater than 99% of theoretical, and the composite material of the backing plate is 30% lighter than Cu while also having higher stiffness than Cu. The high-modulus fiber metal matrix composite backing plate can be used for high power W, Ta, and ceramic PVD targets.

RELATED APPLICATION DATA

[0001] This application is related to U.S. Provisional application No.60/208657, which was filed May 31, 2000.

TECHNICAL FIELD

[0002] This invention relates to high-modulus fiber metal matrixcomposites for use as backing plates with physical vapor depositiontargets and for use in constructions with semiconductor substrates. Theinvention also pertains to utilization of high-modulus fiber metalmatrix composites in packaging applications, with the composites beingused as, for example lids, heat spreaders and heat sinks.

BACKGROUND OF THE INVENTION

[0003] A normal PVD target assembly comprises a target and a backingplate. The target can be joined to the backing plate using mechanical,epoxy, solder or solid state diffusion. General requirements for thejoint are high electrical and thermal conductivity, little or nodistortion during sputtering, and good mechanical support. Mechanicaljoining provides a convenient mechanical support, but does not have goodthermal and electrical conductivity, and so finds only limitedapplication. Epoxy joining is easy, but an epoxy joint does not usuallyhave good thermal and electrical properties. Solder and diffusion arethe two most widely used methods for joining a target to a backingplate.

[0004] General requirements for the backing plate are high thermal andelectrical conductivity, good mechanical strength, and most importantly,a coefficient of thermal expansion that closely matches the coefficientof thermal expansion of the target material. Target materials cancomprise metal, consist essentially of metal, or consist of metal; andcan comprise, for example, one or more of aluminum, copper, titanium,tungsten, tantalum, gold, and alloys thereof. Target materials can also,or alternatively, consist of, consist essentially of, or comprise,ceramic materials, such as, for example, lead, zirconate, and titanate(PZT); lead, lanthanum, zirconate, and titanate (PLZT); strontium bariumtantalate (SBT); and barium strontium titanate (BST).

[0005] Materials such as aluminum, copper and molybdenum have been usedas backing plate materials, but all have drawbacks due to either poormatch to the physical properties of the target material or excessivecost. It would therefore be desirable to develop alternative backingplate materials.

SUMMARY OF THE INVENTION

[0006] The invention includes metal matrix composite backing materialsfor physical vapor deposition targets and for use in constructions withsemiconductor substrates. The metal matrix composite backing materialsof the present invention, can be used as, for example, microelectronicspackaging lids, heat spreaders, and heat sinks. The metal matrixcomposite materials are constructed using a mixture of metal powder (Cu,Al, Ni, Ag, Ti, Co or an alloy of Cu, Al, Ni, Ag, Ti, Co) ranging insize from about −325 mesh size to about 100 mesh size, and discontinuoushigh-modulus material fibers, such as carbon, SiC, SiN, AlO, TiN, B, orcombinations thereof, with fiber lengths ranging from about 10 micronsto about 10 millimeters, and diameters ranging from about 1 micron toabout 25 microns. It is noted that the listed fiber material compoundsare described in terms of the materials in the compounds, rather in aspecific stoichiometry. Thus, for example, the listed AlO can be Al₂O₃.

[0007] Depending on the application, the metal matrix compositematerials can have a composition ranging from 1% by volume fiber to 70%by volume fiber. The materials can be consolidated by use of an axialloading method such as vacuum hot-press (preferred method), hot press,or squeeze casting. The terms “hot press” and “vacuum hot press” referto processes in which powdered metal is compressed to form a structurewithout melting of the metal, and the term “squeeze casting” refers to aprocess wherein a molten metal is solidified under pressure to form astructure. When the metal material is copper and the fibers are carbon,the consolidated materials can have a coefficient of thermal expansionthat ranges from about 3×10⁻⁶/° C. to about 17×10⁻⁶/° C.; depending oncarbon fiber volume and alignment. Thermal conductivity can range fromabout 130 W/mK (watts/meter-Kelvin) to over 400 W/mK, depending on, forexample, carbon fiber volume and alignment. The density of the compositematerial can range from about 4.9 g/cc (gram/cubic centimeter) to about7.6 g/cc.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

[0009]FIG. 1 shows a diagrammatic, cross-sectional view of a backingplate at a preliminary processing step of methodology encompassed by thepresent invention.

[0010]FIG. 2 is a view of the FIG. 1 backing plate shown at a processingstep subsequent to that of FIG. 1.

[0011]FIG. 3 shows a diagrammatic, cross-sectional side view of atarget/backing plate construction formed in accordance with methodologyof the present invention. The construction corresponds to a largeENDURA™ configuration.

[0012]FIG. 4 is a top view of the target/backing plate construction ofFIG. 3.

[0013]FIGS. 5A and 5B show scanning electron microscope images of amaterial encompassed by the present invention, and specifically show thealignment of the carbon fibers in relation to a plan view (FIG. 5A) andcross sectional view (FIG. 5B).

[0014]FIG. 6 is a diagrammatic cross-sectional view of a semiconductorpackage comprising components encompassed by the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The invention encompasses a new material comprising carbon fibersdispersed in a metal matrix. The material can be used, for example, as abacking plate in a backing plate/target construction. In forming backingplate/target constructions, it is found that it is desirable to matchthe coefficient of thermal expansion (CTE) of the backing plate to thatof the target material. If the CTE is not well matched, largethermally-induced stress can occur in the target backing plate assembly.Because of large CTE mismatch, low strength indium solder bonds havebeen used in conventional backing plate/target constructions to helprelieve stress when the bond material deforms during thermal cycling. Ifhigh strength bonds are used instead of the low strength bonds, largedeformation can occur in the backing plate/target construction. In theworst case with targets fabricated from brittle material, the targetwill fracture during thermal cycling.

[0016] One aspect of the present invention is a recognition that anengineered material, such as a high-modulus fiber metal matrix composite(MMC), can meet the requirements of a backing plate in a backingplate/target construction. For purposes of interpreting this disclosureand the claims that follow, a high-modulus fiber material is defined asa fiber material having a modulus higher than the metal matrix,preferably greater than 5 times higher than the metal matrix. Forexample, because the CTE of carbon fiber is extremely low (in fact,negative in the axial direction) it can be introduced into a metalmatrix to cause a reduction in the overall CTE of the bulk material. Theamount of reduction in CTE depends upon the volume fraction of carbonfiber and the alignment of the fibers. The fiber may be aligned in sucha way as to cause the CTE reduction in only one direction or in only oneplane.

[0017] MMC materials of the present invention can have otherapplications besides the above-discussed application as backing platesin backing plate/target constructions. For instance, thethermal-mechanical tunability of MMC materials can also make thematerials excellent candidates for applications in microelectronicspackaging. In applications such as lids, the MMC can be adjusted toapproximately match the CTE of the package, thus improving reliability.For heat spreaders or heat sinks, not only can the CTE be controlledwith MMC materials, but the directionality of the fibers within thematerials can be controlled to control heat flow through the part.

[0018] In one aspect of the invention, 10 micron diameter carbon fibersof 200 micron average length are plated with approximately 30 weightpercent of copper. The copper plated carbon fibers are mixed with adetermined weight of −325 mesh copper powder such that the desiredvolume percent of carbon fiber is achieved. The copper plated carbonfiber/copper powder mixture is then vacuum hot pressed into a desiredshape. The vacuum hot-pressing can occur at a temperature of from about800° C. to about 1000° C., and at a pressure range of from about 4000psi to about 6000 psi.

[0019]FIGS. 1 and 2 illustrate an effect of hot-pressing of adisc-shaped backing plate 10. The plate 10 is shown in cross-sectionalside view in the preliminary processing step of FIG. 1, and carbonfibers 16 are visible extending throughout a metal matrix material ofthe plate 10. At the initial processing step of FIG. 1, the fibers aredistributed about substantially random orientations. The backing plate10 is shown comprising an upper surface 14.

[0020]FIG. 2 illustrates the backing plate of FIG. 1 as a pressing forceis directed against the upper surface 14. Specifically the pressingforce is illustrated by arrows 12 directed against surface 14. Thepressing force can be generated by, for example, a hot press. Such pressapplies an axial pressure to pressed surface 14. As a result of theaxial pressure, carbon fibers 16 align themselves parallel to the axison which the force is applied. When the fibers are so aligned, thecoefficient of thermal expansion is reduced in a plane aligned with thefibers. In disc-shaped plate 10, for example, the fibers are alignedparallel to the plane of the surface 14 of the disc after pressing onthe surface of the disc. Accordingly, the CTE in the directions parallelto the surface is greatly reduced.

[0021] The backing plate CTE can be adjusted by changing the volumepercent of carbon fiber in the composite material to approximately matchthe CTE of the target material, or in particular cases to even exactlymatch the CTE of the target material. When the CTE of the backing plateand the CTE of the target are matched, thermally induced stress can beeliminated from an interface between the two. The result can be a strongand reliable backing plate/target assembly, which can withstand thedemands of high power sputtering.

[0022] A backing plate, or other MMC component formed in accordance withthe present invention, will preferably have good thermal conductivityfor heat dissipation. In the case of the MMC backing plate of thepresent invention, the thermal conductivity is reduced through thehorizontal thickness, but is unchanged in the vertical cross-sectionalplane aligned with the fibers. This effectively causes the MMC backingplate to act as a heat spreader, further reducing the possibility ofthermal stress in the backing plate/target assembly.

[0023] A backing plate, or other MMC component formed in accordance withthe present invention, will preferably have good mechanical strength.Compared to copper the MMC material is stiffer, and more than 30%lighter, which can be a significant improvement for larger parts.

[0024] An exemplary backing plate/target assembly encompassed by thepresent invention is shown in FIGS. 3 and 4 as assembly 50. Assembly 50comprises a backing plate 52 bonded to a target 54. Backing plate 52 andtarget 54 join at an interface 56, which can comprise, for example, adiffusion bond between the backing plate and target; or which cancomprise, for example, a solder bond. Backing plate 52 can comprise anMMC construction, such as, for example, a copper matrix having carbonfibers distributed therethrough. Target 54 can comprise, for example, atungsten target.

EXAMPLE 1

[0025] Milled carbon fiber with an average length of 200 um and adiameter of 10 um coated with 30 weight percent Cu is mixed with −325mesh Cu powder to achieve volume loading of 50 percent carbon fiber. Themixture is blended for three hours. The blended mixture is placed into agraphite hot press die that was prepared by lining the die with spray-onboron nitride. The loaded die assembly is placed and properly aligned ina hot press chamber; five tons of pressure is applied; and the chamberis vacuumed down to 1.5×10⁻³ torr. Upon reaching the called-for vacuum,the temperature is ramped up to 875° C. at a rate of 300° C./hour. Oncea desired temperature is reached, the pressure is increased to 4500 psiat a rate of 10 tons/minute. The temperature of 875° C. is maintainedfor about one hour, at which time the chamber is back-filled with argonto a pressure of 507 torr, and the temperature is ramped down to 300° C.Pressure is maintained at 4500 psi until the temperature reaches 300°C.; then the pressure is released and the heater is turned off.

[0026] The part resulting from the aforementioned process has a measureddensity of 5.56 g/cc, which is 99.5% of the theoretical value of 5.59g/cc. Metalography and SEM imaging of polished samples show an evendistribution of the fibers throughout the matrix, and also show that thefibers are aligned perpendicular to the axis on which the formingpressure was applied. Additionally, it is found that the fibers arerandomly aligned within their plane of alignment. FIGS. 5A and 5B showscanning electron microscope images illustrating the alignment of carbonfibers in relation to a plan view and cross sectional view of the carbonfibers within the matrix.

[0027] The coefficient of thermal expansion in the longitudinaldirection is 4.9×10⁻⁶/° C., and in the transverse direction is 17×10⁻⁶/°C. The thermal conductivity in the longitudinal direction is 300 W/mK,and in the transverse direction is 130 W/mK. The experiment wasrepeated, and the results were duplicated.

EXAMPLE 2

[0028] Milled carbon fiber with an average length of 200 um and adiameter of 10 um coated with 30 weight percent Ni is mixed with −325mesh Cu powder to achieve volume loading of 50 percent carbon fiber. Themixture is blended for three hours. The blended mixture is placed into agraphite hot press die that was prepared by lining the die with spray-onboron nitride. The loaded die assembly is placed and properly aligned inthe hot press chamber, five tons of pressure is applied, and the chamberis vacuumed down to 1.5×10⁻³ torr. Upon reaching the desired vacuum, thetemperature is ramped up to 875° C. at a rate of 300° C./hour. Oncetemperature is reached, the pressure is increased to 4500 psi at a rateof 10 tons/minute. The temperature of 875° C. is maintained for onehour, at which time the chamber is back-filled with argon to a pressureof 507 torr and the temperature is ramped down to 300° C. The pressureis maintained at 4500 psi until the temperature reaches 300° C., andthen the pressure is released and the heater is turned off.

[0029] The part resulting from the aforementioned process has a measureddensity of 5.57 g/cc, which is 99.6% of the theoretical value of 5.59g/cc. Metalography and SEM imaging of polished samples showed an evendistribution of the fibers throughout the matrix, and also shows thatthe fibers are aligned perpendicular to the axis on which the formingpressure was applied. The fibers were randomly aligned within theirplane of alignment. The coefficient of thermal expansion in thelongitudinal direction is 4.8×10⁻⁶/° C. and in the transverse directionis 17×10⁻⁶/° C. The thermal conductivity in the longitudinal directionis 91.73 W/mK and in the transverse direction is 39.75 W/mK.

[0030] The experiment was repeated, and the results presented wereduplicated.

EXAMPLE 3

[0031] Milled carbon fiber with an average length of 200 um and adiameter of 10 um coated with 30 weight percent Cu is mixed with −325mesh Cu powder to achieve volume loading of 40 percent carbon fiber. Themixture is blended for three hours. The blended mixture is placed into agraphite hot press die that was prepared by lining the die with spray-onboron nitride. The loaded die assembly is placed and properly aligned inthe hot press chamber, five tons of pressure is applied, and the chamberis vacuumed down to 1.5×10⁻³ torr. Upon reaching the desired vacuum, thetemperature is ramped up to 875° C. at a rate of 300° C./hour. Once thedesired temperature is reached, the pressure is increased to 4500 psi ata rate of 10 tons/minute. The temperature of 875° C. is maintained forabout one hour, at which time the chamber is back-filled with argon to apressure of 507 torr, and the temperature is ramped down to 300° C. Thepressure is maintained at 4500 psi until the temperature reaches 300°C., and then the pressure is released and the heater is turned off.

[0032] The part resulting from the aforementioned process has a measureddensity of 5.78 g/cc, which is 99.6% of the theoretical value of 5.8g/cc. Metalography and SEM imaging of polished samples showed an evendistribution of the fibers throughout the matrix; that the fibers arealigned perpendicular to the axis on which the forming pressure wasapplied; and that within their plane of alignment the fibers wererandomly aligned. The coefficient of thermal expansion in thelongitudinal direction is 6.3×10⁻⁶/° C. and in the transverse directionis 17×10⁻⁶/° C. The thermal conductivity in the longitudinal directionis 370 W/mK and in the transverse direction is 130 W/mK. The experimentwas repeated, and the results presented were duplicated.

[0033] Although the invention is described above with reference tobacking plate/target assemblies, it is to be understood thathigh-modulus fiber metal matrix composite materials of the presentinvention (i.e., materials in which fibers are dispersed in a metalmatrix, and in which the fibers have a modulus at least about five-timesgreater than that of the metal matrix) can be utilized in otherembodiments of the invention. For instance, FIG. 6 illustrates asemiconductor package 500. The package comprises a semiconductor chip(or die) 502 connected to a substrate 504 (which can be, for example, acircuit board) through electrical interconnects 506 (which can comprise,for example, solder beads). Chip 502 can be considered to be asemiconductor substrate, with the term “semiconductor substrate” definedto mean any construction comprising semiconductive material, including,but not limited to, bulk semiconductive materials such as asemiconductive wafer (either alone or in assemblies comprising othermaterials thereon), and semiconductive material layers (either alone orin assemblies comprising other materials). The term “substrate” refersto any supporting structure, including, but not limited to, thesemiconductive substrates described above.

[0034] A lid 508 is provided over substrate 504 and chip 502 to form aprotective cover over the chip. Lid 508 can comprise a high-modulusfiber metal matrix composite material of the present invention, and canbe referred to as a microelectronics packaging lid. An exemplary metalmatrix of lid 508 is copper or aluminum, and an exemplary high modulusfiber is a carbon fiber. Lid 508 can comprise a metal matrix having highmodulus fibers dispersed therein, consist essentially of a metal matrixhaving high modulus fibers dispersed therein, or consist of a metalmatrix having high modulus fibers dispersed therein.

[0035] In the shown embodiment, lid 508 is thermally connected to chip502 through a thermal interface material 510. Thermal interface material510 enables heat to efficiently pass from chip 502 to lid 508. Lid 508can then dissipate the heat. In the shown embodiment, lid 508 issubstantially planar, and accordingly dissipates the heat primarily intwo dimensions, instead of three. Lid 508 can thus be considered a heatspreader. If lid 508 was constructed to dissipate the heat in threedimensions, it would be a heat sink. It is noted that the shown thermalinterface material 510 can be omitted to leave an air gap, or replacedby a non-thermally conductive material. In either event, heat would nolonger efficiently pass from chip 502 to lid 508, so the lid would onlybe a microelectronics packaging lid, and not a heat spreader.Alternatively, thermal interface material 510 can be omitted and lid 508can be placed directly on semiconductor substrate 502. In suchalternative construction, heat may, in some embodiments, passefficiently to lid 508 so that the lid is a heat spreader; and in otherembodiments the heat may not pass efficiently so that lid 508 is not aneffective heat spreader.

[0036] A thermally conductive interface material 512 is provided overlid 508, and a heat sink 514 is over interface material 512. Interface512 can comprise, for example, a GELVET™ material (available fromHoneywell International, Inc.™), and heat sink 514 can comprise, forexample, a high-modulus fiber metal matrix composite material of thepresent invention. An exemplary metal matrix of heat sink 514 is copperor aluminum, and an exemplary high modulus fiber is a carbon fiber. Heatsink 514 can comprise a metal matrix having high modulus fibersdispersed therein, consist essentially of a metal matrix having highmodulus fibers dispersed therein, or consist of a metal matrix havinghigh modulus fibers dispersed therein.

1. A backing plate material comprised of a metal matrix and fibersdispersed within the metal matrix, the fibers having a higher modulusthan the metal matrix.
 2. The backing plate material of claim 1 whereinthe modulus of the fibers is at least five-times higher than the modulusof the metal matrix.
 3. The backing plate material of claim 1 whereinthe metal of the metal matrix comprises one or more of Cu, Al, Ni, Ag,Ti, and Co.
 4. The backing plate material of claim 1 wherein the fibershave a diameter of from about 1 micron to about 25 microns, and a lengthof from about 10 microns to about 10 millimeters.
 5. The backing platematerial of claim 1 wherein the fibers have a coefficient of thermalexpansion of less than 10×10⁻⁶/° C.
 6. The backing plate of claim 1comprising from about 1% to about 70%, by volume, of the fibers withinthe metal matrix.
 7. The backing plate material of claim 1 wherein thefibers comprise one or more materials selected from the group consistingof SiC, SiN, AlO, TiN, B, and elemental carbon; with the compounds SiC,SiN, AlO and TiN being described in terms of components of the compoundsrather than stoichiometry.
 8. A method of forming a high-modulus fibermetal matrix composite material, comprising, subjecting a mixture offiber and metal powder to one or more hot pressing, vacuum hot pressingor squeeze casting to consolidate the mixture into the high-modulusfiber metal matrix composite material.
 9. The method of claim 8 whereinthe coefficient of thermal expansion of the metal matrix composite isbetween 2×10⁻⁶/° C. and 25×10⁻⁶/° C.
 10. The method of claim 8 whereinthe high-modulus fiber metal matrix composite material has a density ofat least 98% of a theoretical density of the mixture.
 11. Amicroelectronics packaging lid comprising a metal matrix and fibersdispersed within the metal matrix; the fibers having a higher modulusthan the metal matrix.
 12. The microelectronics packaging lid of claim11 wherein the metal matrix comprises one or more of Cu, Al, Ni, Ag, Ti,and Co.
 13. The microelectronics packaging lid of claim 11 wherein themodulus of the fibers is at least five-times higher than that of themetal matrix.
 14. The microelectronics packaging lid of claim 11 whereinthe metal matrix comprises one or more of Cu, Al, Ni, Ag, Ti, and Co;and wherein the fibers are carbon-containing fibers.
 15. A heat spreadercomprising a metal matrix and fibers dispersed within the metal matrix;the fibers having a higher modulus than the metal matrix.
 16. The heatspreader of claim 15 wherein the modulus of the fibers is at leastfive-times higher than that of the metal matrix.
 17. The heat spreaderof claim 15 wherein the metal matrix comprises one or more of Cu, Al,Ni, Ag, Ti and Co.
 18. The heat spreader of claim 15 wherein the metalmatrix comprises one or more of Cu, Al, Ni, Ag, Ti and Co; and whereinthe fibers are carbon-containing fibers.
 19. A heat sink comprising ametal matrix and fibers dispersed within the metal matrix; the fibershaving a higher modulus than the metal matrix.
 20. The heat sink ofclaim 19 wherein the modulus of the fibers is at least five-times higherthan that of the metal matrix.
 21. The heat sink of claim 19 wherein themetal matrix comprises one or more of Cu, Al, Ni, Ag, Ti and Co.
 22. Theheat sink of claim 19 wherein the metal matrix comprises one or more ofCu, Al, Ni, Ag, Ti and Co; and wherein the fibers are carbon-containingfibers.
 23. An assembly comprising a ceramic material target bonded to abacking plate; the backing plate comprising: metal; and fibers dispersedwithin the metal and having a higher-modulus than the metal.
 24. Theassembly of claim 23 wherein the modulus of the fibers is at leastfive-times higher than the modulus of the metal.
 25. The assembly ofclaim 23 wherein the ceramic material of the target is selected from thegroup consisting of PZT, PLZT, BST and SBT.
 26. The assembly of claim 23wherein the metal of the backing plate comprises copper.
 27. Theassembly of claim 23 wherein the metal of the backing plate consists ofcopper.
 28. The assembly of claim 23 wherein the metal of the backingplate consists essentially of copper.
 29. The assembly of claim 23wherein the metal of the backing plate comprises copper and wherein thefiber consists essentially of one or more materials selected from thegroup consisting of carbon, SiC, SiN, AlO, TiN, B; with the compoundsSiC, SiN, AlO and TiN being described in terms of components of thecompounds rather than stoichiometry.
 30. An assembly comprising a firstmaterial target bonded to a second material backing plate; the secondmaterial being different than the first material; the second materialcomprising metal and fibers dispersed within the metal; the fibershaving a modulus that is at least five-times larger than a modulus ofthe metal; the relative amount of fibers and metal in the secondmaterial being configured to provide the second material with about thesame coefficient of thermal expansion as the first material.
 31. Theassembly of claim 30 wherein the first material comprises one or more ofaluminum, copper, titanium, tungsten, tantalum and gold.
 32. Theassembly of claim 30 wherein the second material comprises copper metaland carbon fibers dispersed within the copper metal.
 33. The assembly ofclaim 30 wherein the second material consists essentially of coppermetal and carbon fibers dispersed within the copper metal.
 34. Theassembly of claim 30 wherein the second material consists of coppermetal and carbon fibers dispersed within the copper metal.
 35. A methodof forming a physical vapor deposition target assembly, comprising:forming backing plate from metal and fibers, the fibers having a modulusthat is at least five-times larger than a modulus of the metal; thebacking plate having a surface which is to be bound to the target, thefibers being aligned substantially perpendicular to said surface; andbonding the backing plate surface to a physical vapor deposition targetto form the sputtering target assembly.
 36. The method of claim 35wherein the forming the backing plate comprises forming the backingplate from copper metal and carbon fibers.
 37. The method of claim 35wherein the physical vapor deposition target comprises ceramicmaterials.
 38. The method of claim 35 wherein the physical vapordeposition target comprises one or more of aluminum, copper, titanium,tungsten, tantalum, and gold.